1. Field of the Invention
The present invention is directed to highway finishing machines and, more particularly, to a double tamping bar vibratory screed for laying a roadway which is capable of being affixed to a conventional piece of lay-down equipment and can run at higher speeds while providing an extremely high compaction density, especially with high density materials.
2. Description of the Related Art
Highway finishing machines are well known in the art and generally consist of a lay-down machine which accepts the material that is to form the roadbed, distributes it across the desired width of roadway to be laid and compacts the material, usually with a vibrating plate, to a desired compaction rate and finish. These machines have been found to be effective for laying some materials, such as asphalt, as well as some types of conventional concrete.
Recently, highway construction companies have found it very desirable to lay road mixes having higher densities in order to reduce wear on the roadways and thereby increase their life and to accommodate traffic of heavy vehicles and equipment. These higher densities are normally provided in concrete applications by increasing the level of aggregate within the concrete mix itself. One form of high density concrete is known as roller-compacted concrete which is known as a zero-slump concrete mixture and which has been basically used for low-volume, low speed roadways having vehicle traffic with high axial loads.
Due to its increased density, roller-compacted concrete has been very difficult to lay with conventional road paving machinery. This is due to its lack of plasticity which prevents it from flowing into a smooth even surface and inhibits proper compacting. A smooth finish is necessary for roadways, especially where a higher vehicle speed is desired, in order to prevent excess tire wear and reduce noise. Proper compacting is necessary in order to prevent breakdown of the roadway, especially when heavy equipment and vehicles are used.
Roadway laying machines have been developed for laying this type of high density or roller-compacted concrete. One type of such a machine is disclosed in U.S. Pat. No. 4,507,014 issued to Heims et al which discloses a highway finishing machine having at least two stamps which have a common eccentric support. These two stamps cooperatively slidingly engage with one another and have different vertical lengths so that they reciprocate at two slightly different levels.
Major disadvantages with existing highway paving machines, especially when used for laying high density materials such as roller-compacted concrete, are their relatively slow linear speeds, their inability to adapt to different materials and consistencies and their tendency to break up or tear the material being applied. Therefore, the process becomes very time consuming and creates the need for further compacting and surface treatment after the initial layer is applied.